Current transformer



Feb. 15, 1944. A. T. SINKS CURRENT TRANSFORMER 2 Shets-Sheet 1 OriginalFiled June 6, 1940 :s a Pk n m w m S j t eT c Vn A g W6 Patented Feb.15, 1944 CURRENT TRANSFORMER Allen '1. Sinks, Beach Blufl, Masa.assignor to General Electric Company, a corporation of New York Originalapplication June 6, 1940, Serial No. 339,121. Divided and thisapplication March 4, 1941, Serial No. 381,689

6 Claims.

My invention relates to transformers and to current transformers for usein connection with differential protective systems foralternatingcurrent power systems.

This application is a division of my application, Serial No. 339,121,filed June 6, 1940, entitled Electric protective arrangement, andassigned to the same assignee as the present application.

Differential protective arrangements usually include currenttransformers for deriving, for the control of the protective apparatus,currents which are respectively proportional to the magnitudes of thecurrents at difierent points on the system or portion thereof beingprotected. For this purpose, it has been the practice in protectivearrangements for altemating-current power systems to use currenttransformers having closed magnetic cores. However, the use of suchclosed magnetic-core current transformers in protective arrangements forpower systems, which embody highly inductive rotating apparatus andwhich are subject to sudden changes of current under certain conditions,such as fault conditions, for example, has resulted in false operationof the relays, particularly relays of the type in which there isproduced a torque dependent solely upon the magnitude of the current ina single winding of the relay, of the protective arrangement because ofthe saturation of the cores of the current transformers by thedirect-current transient com ponent which is superimposed on thesymmetrical alternating-current component of current.

Designers of alternating-current protective arrangements have beenconfronted with this problem for many years and various arrangementshave been suggested to prevent such false operation of the protectiverelays due to the saturation of the associated current transformers bysuch direct current transient component of the current, but all of theseproposed arrangements have certain disadvantages which have preventedthem from becoming an entirely satisfactory practical solution of theproblem.

One suggested arrangement for overcoming this difliculty was to usecurrent transformers having closed magnetic cores of suiiicient size toprevent them from becoming saturated under all current conditions thatcan occur in the power circuit or the portion thereof being protected.

The size and cost of such transformers for use in power circuitscarrying large currents are such, however, that it is impractical to usethem.

Another suggested arrangement was to use current transformers havingnonmagnetic cores. Such current transformers, however, have such a smallsecondary output that it is necessary to provide, in addition, suitablecurrent amplifying means so that enough current can be obtained tooperate the protective relays. Such additional means not only increasesthe cost of the apparatus but results in a very complicated arrangement,particularly when there are a large number of parallel connected currenttransformers involved, as is the case in bus protective arrangements.

A third arrangement, which has been suggested for certain types ofalternating-=current protective systems, consists of a shunt circuitaround the current Winding of each protective relay. This shunt circuitis designed so that the direct-current component of the current inducedin the current transformer secondary winding flows through the shuntcircuit instead of through the relay winding in order that only thealternating-current component of the current can flow through the relaywinding. Since this also requires additional apparatus for eachtransformer, it results in an expensive and complicated arrangement.

Also, in order to avoid false operation due to the saturation effectproduced by the directcurrent transient component, it has sometimes beennecessary to raise the settings of the pro tective relays, whichseriously impairs the proper functioning of the protective arrangementunder certain fault conditions.

It is an object of my invention, therefore, to provide a new andimproved current transformer for use in connection with analternating-cur rent protective system.

A further object of my invention is to provide an improvedalternating-current protective system embodying current transformerswhereby false operation of the protective relays due to thedirect-current transient component of the current is prevented withoutthe necessity of providing additional auxiliary apparatus such ascurrent-amplifying means or auxiliary shunt circuits around the currentcoils of the protective relays and without impairing the properfunctioning of the relays in response to all fault currents.

In acordance with my invention, I accomplish the desired result ofpreventing the current in the power circuit being protected from causinga false operation of the protective relays by employing currenttransformers having magnetic cores which have one or more air gapstherein so that the cores do not become saturated under any currentcondition occurring in the circuit or portion thereof being protected.With such an arrangement, not only is there a linear relationshipmaintained between the primary and sec-' ondary currents of eachtransformer under all current conditions occurring in the power cir cuitbeing protected but also, by properly designing the core, suflicientsecondary current for operating the protective relays can be obtaineddirectly from the current transformer under all fault conditionsrequiring such operation.

For a better understanding of my invention, reference may be had to theaccompanying drawings in which Fig. 1 is a schematic diagram of a,differential protective system embodying my invention, Fig. 2 is a frontview partly in section of a current transformer which may be used inconnection with the protective system of Fig. 1, Fig. 3 is a side viewpartly in section of the core and windings of the current transformer ofFig. 2, Fig. 4 is a top view of Fig. 3, Fig. 5 is a side view of Fig. 2,and Fig. 6 is a schematic diagram of a portion of a protective systemalso embodying my invention.

It will be understood by those skilled in the art that my invention isapplicable to many different protective arrangements and, by way ofexample, I have chosen to illustrate my invention in Fig. 1 as appliedto a differential protecti ve system for a polyphase alternating-currentsectionalized bus system of which three sections it, H, and i2 areschematically shown. Only one section others are substantiallyduplicates as far as my invention is concerned and this section it isillustrated as a three-phase bus including phase conductors Ha, H b, andHo, The bus sections H], H, and B2 are shown as interconnected bysuitable switching means, such as latched closed circuit breakers i3 andI i, each provided with trip coils l5.

Each bus section, such as I l, for example, may have one or more sourcesof supply, which I have indicated as a V-connected generator 96 providedwith a grounded neutral connection ll. In order to disconnect source itfrom bus section ii in case of a fault, a suitable switching means, suchas circuit breaker i8, is provided having a trip coil l9.

A plurality of feeders, such as 20 and 2!, are connected to bus sectionI] through suitable switching means, such as latched closed circuitbreakers 22, which are provided with trip coils 23.

In order to isolate a faulty bus section, I provide means for effectingthe opening of the bus bar circuit breakers l3 and M, the source circuitbreaker l8, and all the feeder circuit breakers 22 upon the occurrenceof a fault on the protected section II. As shown, this means comprisesphase-fault differential relays 26a, 24b, and 2 80 and a ground-faultdifferential relay 25. The particular construction of these relays formsno part of the present invention and, hence, they are only schematicallyshown. These relays are arranged to be energized by a current dependenton the vector sum of all the currents flowing into and out of the bussection II being protected. This differential relay current is obtainedfrom current transformers 26 located at the ends of the bus sections. inthe generator circuit, and in the feeder circuits 2!! and 21 connectedto bus section II. In view of the three-phase system, the secondarywindings of these current transis shown completely since the formers 26at any point are illustrated as connected in Y and these Y-connectedgroups are connected in parallel, phase by phase, through conductors27a, 21b, and 210, respectively, and a common return conductor 28. Inorder to provide an equal burden for all the current transformers, theimpedances of the respective circuits including the secondary windingsof each of the current transformers 26 are preferably arranged to havethe same value. Furthermore, since the lead lengths to the relays vary,a low secondary ampere rating is desirable in order to facilitatebalancing the burdens.

The phase-fault difierential relays 24a, 24b, and 240 are energized inparallel across one of the respective conductors 27a, 21b, and 21c andthe return conductor 28 associated with the neutrals of the Y-connectedcurrent transformers 26. The ground-fault differential relay 25, on theother hand, is connected in series with the parallel arrangedphase-fault relays. Each of the relays, when energized, closes orbridges a pair of contacts 29 so as to energize the tripping coils i5,i9, and 23 to trip the associated circuit breakers and isolate bussection ii.

The differential protective system so far described is similar to thedifferential protective systems of the prior artin-whichcurrenttransformers of the conventional closed magnetic-core type were used.

For reasons which are well known in the art, the direct-currentcomponent of the current on through faults, for example, in conjunctionwith the alternating-current component in such differential protectivesystems effected unequal saturation of the various current transformersso that, although a balance existed between the power currents into andout of the particular portion of the power circuit being protected,there was not a corresponding balance between the secondary currents ofthe current transformers. Consequently, a difierential current ofsufficient value flowed through the protective relay windings to effectan undesired operation of the relays so that a false tripping of thecircuit breakers 22 was effected to isolate th bus section it.

To overcome the difiicult due to this directcurrent component ofcurrent, I employ, in accordance with my invention, magnetic-corecurrent transformers 26 which are designed so that the cores are notsaturated by the maximum currents that can flow through their respectiveprimary windings and so that sumcient current also can be obtained fromtheir secondary windings to operate the protective relays directly underall fault conditions requiring such operation. These results areobtained by providing the current transformer cores with one or moresuitable air gaps.

In Figs. 2, 3, and 5, there is disclosed in detail 9. currenttransformer having a magnetic core provided with One or more air gapswhich may be used in the differential protective system disclosed inFig. 1. This current transformer is illustrated as a bar-typetransformer having a straight bar or conductor 32 for the primarywinding around which is arranged the core 30 provided with one or moreair gaps 3i.

The core generally designated at 30 comprises a plurality of shortlaminated sections 33 and 34 arranged in a particular manner to form arectangle or closed core member around bar conductor 32. The ends oftheselaminated sections are suitably spaced to form a plurality of airgaps 3|. In order to hold these core sections in place,

two sets of core clamps and 36, respectively, are provided ofnonmagnetic material. The core clamps 35 are provided to hold thelaminated core sections 33 of the vertical legs in proper position.These clamps include extensions 35a at the lower ends thereof to formsupporting or mounting legs for the current transformer. Core clamps 36hold laminated core sections 34 in position to form the upper and lowerlegs of the current transformer core generally indicated at 30. Thelaminated sections 33 and 34 are fastened to the core clamps 35 and 36,respectively, by suitable rivets 31 which are preferably constructed ofsteel in order to withstand the mechanical forces involved underhigh-current conditions.

Since the plurality of air gaps 3| arranged around the rectangular core3|] are relatively large with respect to the cross section of themagnetic core, the fringing flux adjacent thes air gaps is likely to bea large percentage of the total flux, thus tending to caues the outsideof the core section to operate at much higher flux densities than theinner portions of the core. In order to substantially eliminate thistendency and to operate the magnetic core at a more or less uni formdensity, the outer laminations at the gap are arranged in step fashion,that is, the ends of the laminations are staggered with respect to eachother so as to provide portions 38 of one cross section and portions 39of a different cross section. In addition, the ends of the laminationsare preferably rounded at the corners. As shown, the magnetic coresections 34 are produced by using short laminations 40 in combinationwith longer laminations 4| while the core sections 33 are formed bystaggering still longer laminations 42 with respect to one another. Thereason for providing a plurality of air gaps rather than one or two isto decrease the percentage of fringing flux at the gaps as well as toreduce the effect of interference from stray fields.

The air gaps 3| at the corners of the rectangular core structure may bemade adjustable in order to adapt the current transformer for variousapplications. Accordingly, I provide adjusting screws 43, one at eachcorner of the transformer core by means of which the gap length may bevaried. It will be understood, of course, that,

when the current transformers are designed for transformers are designedso that sufficient secondary current is obtained therefrom to operateconventional relays.

In view of the large mechanical stresses involved, it may be desirableto fill the so-called "air gaps 3| with a nonmagnetic material, such asan insulating material of a type able to withstand fairly largemechanical stresses. In Fig. 3, I have shown an insulating material 44inserted in one of the gaps 3| in order to increase the mechanicalstrength of the core to withstand the stresses involved underhigh-current conditions.

The secondary winding of the current transformer comprises four coils,two coils 45 being associated with the upper and lower le s of core 30while coils 45 are associated with the two vertical legs of core 30, andthese four coils are connected in series with one another. Thisparticular disposition of the coils or secondary winding eliminates theeffect of stray fields in so far as the current induced in thesewindings is concerned. As shown best in Figs. 2 and 4, the terminals ofthe secondary windings are supported on a suitable terminal block 41mounted on the current transformer.

It has been found that, even with considerable spacing of the primaryconductors, the leakage or stray flux from adjacent conductors is aconsiderable percentage of the flux in the core of the currenttransformer. As mentioned above, due to the disposition of the coils onthe core, the effect of this stray flux on the secondary winding asidefrom saturation of the core is eliminated. However, this stray fluxwould tend to cause the core of the current transformer to saturatesooner than if this flux were not present and, to substantiallyeliminate this possibility, I provide a suitable shielding means 48around th current transformer which preferably comprises a welded orsoldered copper casing having windows 49 on either side thereof throughwhich the bar conductor 32 may extend. Suitable insulation 50 will, ofcourse, be provided around the primary winding comprising bar conductor32.

When current transformers of the type disclosed in Figs. 2 to 5,inclusive, are used in the differential protective system shown in Fig.1, there is substantially no possibility of effecting false operation ofthe protective relays due to the fact that the current transformers aredesigned to have a linear relationship between the primary and secondarycurrents thereof for all possible primary currents that might exist onthe system. Furthermore, since the same linear relationship ismaintained between the primary and secondary currents in all of thecurrent transformers, no differential current can flow through theprotective relays as long as there is no fault on bus section When,however, a fault does occur on bus section II, the current transformersare designed so that a differential current of sufficient magnitude isproduced which flows through the protective relays to effect operationthereof.

Although I have thus far described my invention as applied to adifferential protective system, false operations of relays associatedwith protective systems other than differential systerns have resultedbecause of the saturation of the cores of the current transformers bythe direct-current transient which is superimposed on the symmetricalalternating-current. Thus, in highly inductive circuits, ordinaryswitching operations might cause a direct-current transient ofsuiiicient magnitude when superimposed upon the symmetricalalternating-current to cause false operation of an ordinary overcurrentrelay, for example, even though no actual abnormal current existedrequiring operation of the protective system. I have found that acurrent transformer provided with a magnetic core including one or moreair gaps may be used in such protective systems not only to function asa true transformer but effectively to eliminate the direct-currentcomponent from causing false operation of the protective relays.

In Fig. 6, I have disclosed a relay 5| whose contacts 52 are connectedin the tripping circuit of a circuit breaker, not shown, associated witha suitable protective system. The winding of relay 5| is connected tothe secondary winding 53 of a magnetic-core current transformer 54provided with one or more air gaps and having a primary winding 55 whichmay be connected to the conventional closed magnetic-core currenttransformer associated with the system to be protected, or, as in Fig.l, current'transformer 54 may be associated directly with a conductor ofthe system to be protected. As was described above, current transformer54 will be designed so that the core thereof does not saturate eventhough a direct-current transient. component is superimposed upon thesymmetrical alternating current impressed on primarywinding 55 and sothat sufiicient secondary current for operating relay 5| is obtainedwhen a predetermined abnormal condition occurs on the protected system.I have found that if a closed magnetic-core current transformer wereused, false operation of relay 5| might result, whereas when amagneticcore current transformer provided with one or more air gaps isused, such false tripping is substantially eliminated and relay 5| isnot subjected tothe saturation and other disturbing effects produced bythe long-time constant direct-current components as was common in theprotective systems of the prior art.

Without intending to be bound by the theory advanced, the followingdiscussion is offered to explain how the effect of thedirect-currentists between the primary and secondary currents thereof.The time constant of the circuit associated with the secondary windingof the current transformer is equal to L/R where L is the inductance ofthis circuit and R the resistance thereof. It will, of course, beunderstood by those skilled in the art that the inductance of a currenttransformer having one or-more air gaps in the magnetic core thereof isconsiderably lower than that of a current transformer havin a closedmagnetic core. Furthermore, since the inductance of the secondarywinding of the current transformer is a large proportion of theinductance of the circuit associated with this winding, the timeconstant of this circuit is very much shorter when a current transformerhaving a magnetic core including an air gap is used than when a closedmagnetic-core current transformer is used. Due to this short-timeconstant. the direct-current transient impressed on the primary windingof the current transformer even though transformed dies out within sucha short interval of time as to have substantially no effect on the relayassociated therewith, whereas, if this secondary circuit had a muchlonger time constant, such as would be the case when a closedmagnetic-core current transformer were used, this direct-currentcomponent reflected in the secondary winding would adversely affect therelay 5| as set forth above and cause false tripping. In order to avoidfalse tripping in the arrangements of the prior art, it was necessary touse relays which would not operate until after this direct-currenttransient had disappeared and such delay obviously adversely affectedthe protection obtainable. However, in the present arrangement, thedirect-current transient refiected in the secondary winding dies outwithin such a short interval of time that high-speed relays maysatisfactorily be employed. Thus, with my arrangement, the effect of thedirectcurrent component on high-speed relays is substantially eliminatedwithout resorting to the use of complicated filter means or the like.

While I have shown and described certain particular embodiments of myinvention, it will be apparent to those skilled in the art that myinvention has other applications and that changes and modifications maybe made without departing from the spirit and scope of my invention andI, therefore, aim in the appended claims to cover all such modificationsand changes.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l.In combination, a current transformer comof spaced laminated sectionsso as to provide a plurality of air gaps around said core to preventsaturation of said core and consequent ratio errors of said transformer,said laminated sections adjacent said air gaps being so constructed andarranged to reduce the fringing fiux so that the magnetic sections ofsaid core are operated at substantially uniform flux density, and aplurality of serially connected windings on said rectangular corepositioned so as to neutralize. the inductive effect of stray fieldsthereon.

3. In combination, a current transformer comprising a rectangular coreincluding a plurality of spaced laminated sections so as to Provide aplurality of air gaps around said core to prevent saturation of saidcore and consequent ratio errors of said transformer, said laminatedsections adjacent said air gaps being arranged to reduce the fringingflux so that the magnetic sections of said core are operated atsubstantially uniform flux density, a plurality of serially connectedwindings on said rectangular core positioned so as toneutralize theinductive efiect of stray fields thereon, and a shielding means for saidtransformer to prevent stray fields from saturating said core.

4. In combination, a current transformer comprising a rectangular coreincluding a plurality of spaced laminated sections so as to provide aplurality of air gaps around said core to prevent saturation of saidcore and consequent ratio errors of said transformer, said laminatedsections adjacent said air gaps being so constructed and arranged toreduce the fringing flux so that the magnetic sections of said core areoperated at substantially uniform flux density, a plurality of seriallyconnected windings on said rectangular core positioned so as toneutralize the inductive effect of stray fields thereon, and means foradjusting the width of said air gaps.

5. In a transformer comprising a closed magnetic core formed of aplurality of magnetic sections and a plurality ofnonmagnetic sectionsbetween said magnetic sections so as to prevent saturation of said coreand consequent ratio errors of said transformer when energized with adirect-current component which is not transformed thereby, saidnonmagnetic sections being formed of a material which will withstand themechanical stresses on said core under abnormal current conditions, andasecondary winding on said core arranged to neutralize substantially theinductive eiiect of stra fields thereon.

6. In a transformer comprising a closed magnetic core formed of aplurality of magnetic sections and a plurality of nonmagnetic sectionsbetween said magnetic sections so as to prevent saturation of said coreand consequent ratio errors of said transiormer when energized with adirect current component which is not transiormed thereby, saidnonmagnetic sections being-formed of a material which will withstand themechanical stresses on said core under abnormal current conditions. asecondary winding on said core arranged to neutralize substantially theinductive effect of stray ilelds thereon, and a conductive casing aroundsaid core and winding so as to substantially prevent stray acids fromsaturating said core.

ALLEN '1'. SINKB.

